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Cron Explainer

Validate and explain schedules
15 2 * * 1-5

At minute 15, hour 2, day-of-month *, month *, day-of-week 1-5.

Educational defensive guidance. Review every result before production use.

Introduction

The Cron Explainer validates a standard five-field cron expression — minute, hour, day of month, month, day of week — and explains what it means in plain language.

Cron syntax is compact and easy to misread: 0 2 * * 1-5 and 0 2 * * 1,5 mean very different things. A second pair of eyes on the schedule prevents both missed jobs and 2 a.m. surprises.

Objective

  • Validate that an expression has exactly five valid fields.
  • Translate the fields into a readable sentence.
  • Offer common presets as a starting point.

Inputs

  • A five-field cron expression, for example 15 2 * * 1-5.
  • Fields: minute (0–59), hour (0–23), day of month (1–31), month (1–12), day of week (0–7, where 0 and 7 are Sunday).

How it works

The expression is split on whitespace. It must yield exactly five fields, and every field must consist only of digits and the operators * / , -.

The tool does not verify numeric ranges deeply (for example 99 as an hour still passes the character check) — the explanation is honest about the five-field structure.

A valid expression produces a sentence that reads each field literally; an invalid one produces a clear error message asking for exactly five fields.

Testable example

Try it — the analysis runs locally in your browser.

Example

Expression: 15 2 * * 1-5

Expected output

Valid.
At minute 15, hour 2, day-of-month *, month *, day-of-week 1-5.
Interpretation: weekdays at 02:15.

Reading the output

  • If the explanation matches what you intended, the schedule is likely right.
  • Watch for the classic trap: day-of-month and day-of-week are OR-combined in standard cron (Vixie), so an expression with both restricted can run more often than expected.
  • Presets (hourly, daily, weekly, every 5 minutes) are safe starting points.

Risks

  • A typo in the minute or hour field shifts a whole schedule — for example 30 2 * * * runs at 02:30, not 02:00.
  • Using both day-of-month and day-of-week fields creates OR semantics that surprise most people.
  • Jobs that run when the system is asleep or down are skipped or delayed depending on cron implementation — anacron and systemd timers behave differently.

Limitations

  • The tool accepts the standard five-field format only; it does not parse Vixie extensions like @reboot or the six-field (seconds) variant.
  • Range checking is shallow by design: 99 in the hour field passes the character check.
  • It explains the expression; it cannot verify that the underlying command exists or has the right permissions.

Official references

FAQ

What does the day-of-week field accept?

0–7, where 0 and 7 are Sunday. Names like MON are not accepted by this validator — stick to numbers, ranges and lists.

Why is my job running more often than I expected?

If both day-of-month and day-of-week are restricted, Vixie cron runs the job when either matches. Use * in one of the two fields to get the intuitive AND behaviour.

Should I use cron or systemd timers?

For new work, systemd timers offer better logging, dependencies and missed-run handling. Cron remains fine for simple, well-understood schedules.